As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased. Among such secondary batteries is a lithium secondary battery exhibiting high energy density and operating voltage and excellent preservation and service-life characteristics, which has been widely used as an energy source for various electronic products as well as mobile devices.
Based on the shape of a battery case, a secondary battery may be classified as a cylindrical battery having an electrode assembly mounted in a cylindrical metal container, a prismatic battery having an electrode assembly mounted in a prismatic metal container, or a pouch-shaped battery having an electrode assembly mounted in a pouch-shaped case formed of an aluminum laminate sheet. The cylindrical battery has advantages in that the cylindrical battery has relatively large capacity and is structurally stable.
The electrode assembly mounted in the battery case serves as a power generating element, having a cathode/separator/anode stack structure, which can be charged and discharged. The electrode assembly may be classified as a jelly roll type electrode assembly configured to have a structure in which a long sheet type cathode and a long sheet type anode, to which active materials are applied, are wound in a state in which a separator is disposed between the cathode and the anode, a stacked type electrode assembly configured to have a structure in which a plurality of cathodes having a predetermined size and a plurality of anodes having a predetermined size are sequentially stacked in a state in which separators are disposed respectively between the cathodes and the anodes, or a stacked/folded type electrode assembly, which is a combination of the a jelly roll type electrode assembly and the stacked type electrode assembly.
In this connection, the structure of a conventional cylindrical secondary battery is shown in FIG. 1.
Referring to FIG. 1, a cylindrical secondary battery 100 is manufactured by mounting a jelly roll type (wound type) electrode assembly 120 in a battery case 130, injecting an electrolyte into the battery case 130, and coupling a cap assembly 140 having an electrode terminal (for example, a cathode terminal, which is not shown) to the upper end, which is open, of the battery case 130.
The electrode assembly 120 is configured to have a structure in which a cathode sheet 121 and an anode sheet 122 are wound in a circle in a state in which a separator 123 is disposed between the cathode sheet 121 and the anode sheet 122. A cylindrical center pin 150 is fitted in the core of the electrode assembly 120 (the center of the jelly roll). The center pin 150 is generally made of a metal material to exhibit predetermined strength. The center pin 150 is configured to have a hollow cylindrical structure formed by rolling a sheet type material. The center pin 150 serves to fix and support the electrode assembly. Also, the center pin 150 serves as a passage to discharge gas generated by internal reaction of the secondary battery when charging and discharging the secondary battery and when operating the secondary battery.
An insulation member 160, which is configured to have a sheet type structure, is mounted at the upper end of the electrode assembly 120. The insulation member 160 is provided at the center thereof with an opening communicating with a through hole 151 of the center pin 150, through which gas can be discharged and through which a cathode tab 142 of the electrode assembly 120 can be connected to a cap plate 145 of the cap assembly 140.
Also, an insulation member 170 is disposed at the lower end of the cylindrical secondary battery 100. The insulation member 170 is located between the lower end of the electrode assembly 120 and the battery case 130. The insulation member 170 is provided at the center thereof with an opening, through which an anode tab (not shown) attached to the anode sheet 122 is connected to the lower end of the battery case 130.
The cathode tab and the anode tab are generally connected to uncoated portions of the cathode sheet and the anode sheet (portions of the electrode sheets, i.e. metal current collectors, to which electrode active materials are not applied) by ultrasonic welding.
Specifically, ultrasonic welding between the cathode sheet and the cathode tab is typically shown in a sectional view of FIG. 2.
Referring to FIG. 2, the cathode tab 142 is disposed on an uncoated portion 121a of the cathode sheet in contact with the uncoated portion 121a of the cathode sheet, and the cathode tab 142 is pressed by an ultrasonic welding device 300 including a plurality of horn tips 310. At this time, ultrasonic vibration is transmitted to the cathode tab 142, and the cathode tab 142 is welded to the uncoated portion 121a of the cathode sheet by frictional heat generated as the result of the ultrasonic vibration.
However, the cathode tab and the cathode sheet are flat with the result that the cathode tab may wear down when ultrasonic welding is performed in a state in which the horn tips are in contact with the cathode tab. Also, high coupling between the cathode tab and the cathode sheet is required, and therefore, electrode active materials may be separated from the cathode sheet during welding based on the application of high ultrasonic waves.
Therefore, there is a high necessity for technology to fundamentally solve the above problems.